This invention relates generally to fuel cell power plants and, more particularly, to a method and apparatus for using an absorption chiller in combination therewith.
A fuel cell is an electrochemical cell which consumes fuel and an oxidant on a continuous basis to generate electrical energy. The fuel is consumed at an anode and the oxidant at a cathode. The anode and cathode are placed in electrochemical communication by an electrolyte. One typical fuel cell employs a phosphoric acid electrolyte. The phosphoric acid fuel cell uses air to provide oxygen as an oxidant to the cathode and uses a hydrogen rich stream to provide hydrogen as a fuel to the anode. After passing through the cell, the depleted air and fuel streams are vented from the system on a continuous basis.
A typical fuel cell power plant comprises one or more stacks of fuel cells, the cells within each stack being connected electrically in series to raise the voltage potential of the stack. A stack may be connected in parallel with other stacks to increase the current generating capability of the power plant. Depending upon the size of the power plant, a stack of fuel cells may comprise a half dozen cells or less, or as many as several hundred cells. Air and fuel are usually fed to the cells by one or more manifolds per stack.
In each of the fuel cells, waste heat is a by-product of the reforming process for conversion of fuel to a hydrogen rich steam, electrochemical reactions and the heat generation associated with current transport within the cell components. Accordingly, a cooling system must be provided for removing the waste heat from a stack of fuel cells so as to maintain the temperature of the cells at a uniform level which is consistent with the properties of the material used in the cells and the operating characteristics of the cells. This has typically been accomplished by circulating a coolant, such as water, through the fuel cell stack to cool the cells to the required level, with the temperature of the water emanating from the fuel stack being relatively high (i.e. over 300° F.). While temperature of 300-350° F. are acceptable for industrial or large campus customers, which are relatively few in number, those temperature are too hot for most main stream commercial customers. Thus, low grade and high grade customer heat exchangers were added to ensure ease of integration with most commercial customer heat transfer equipment. That is, these heat exchangers and cooling loops were added to reduce the temperature down to a useful level (i.e. around 150° F.) for ordinary boiling uses such as boiler feed water, heating coils in the air handling systems, radiant heating, hydraulic heating, laundry and household use. These exchanges also served the purpose of protecting the cell stack from possible contaminants in the customer side loops. Such a design has thus become the standard in the industry, with all fuel cell plants being sold with the associated heat exchangers installed as standard equipment.
One application that has recently come into use is that of applying the hot water available as a low grade heat source to drive absorption chillers for the generation of chilled water for space cooling. While this does make use of the waste heat from the fuel cell power plant, it has been found to be a relatively inefficient use of an absorption chiller. That is, the major drawback with using customer heat exchangers is that the addition of another heat transfer loop leads to thermal losses and driving force losses, as these external loops must operate at lower temperatures.